msm: clock: Setup sibling lists in msm_clock_register()

msm_clock_register() should initialize each clocks' list of
children, as is already done in msm_clock_init(). This allows
the clock-voter.c code to work with clocks registered through
the msm_clock_register() API. Since no clocks are registered
through that API yet, this change should have no immediate
functional effect.

Change-Id: I257999da82615e99da2cf76fdda48f43e975107e
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>